Considerable efforts have been made to increase the sensitivity of silver halide photographic materials. In particular, there has been a strong desire for increasing the sensitivity of spectrally sensitized silver halide photographic materials.
The spectral sensitization art is a very important and essential technique used in producing photographic materials having high sensitivity and excellent color reproducibility. Spectral sensitizers have a function of absorbing light of long wavelengths which silver halide photographic emulsions cannot absorb in themselves and transmitting it to silver halides. Accordingly, an increase in the amount of light caught by spectral sensitizers is advantageous to enhancement of photographic sensitivity. Thus, attempts have been made at increasing the amount of light caught by spectral sensitizers via the increase in amount of spectral sensitizers added to a silver halide emulsion.
However, if they are added to a silver halide emulsion in an amount above the optimum, far from sensitizing, spectral sensitizers cause strong desensitization in the emulsion. This desensitization phenomenon is generally referred to as dye-induced desensitization, and caused by dyes in spectral regions wherein sensitizing dyes show no substantial absorption but silver halides have intrinsic sensitivity. If dyes cause strong desensitization, the sensitivity as a whole is decreased though they have spectral sensitization effect. In other words, the spectral sensitivities are increased in proportion as the desensitization by dyes is diminished. Therefore, the improvement of dye-induced desensitization is an important subject in the spectral sensitization art.
Further, as described in T. Tani, Journal of Physical Chemistry, vol. 94, p. 1298 (1990), the sensitizing dyes having reduction potentials more positive than -1.25 V are known to be low in relative quantum yield of spectral sensitization. For the purpose of heightening the relative quantum yield of spectral sensitization by dyes, it is proposed to carry out the supersensitization by positive hole capture as described in The Theory of the Photographic Process, pp. 259-265 (1966).
Compounds which can serve as a supersensitizer to get rid of the aforementioned desensitization are those having oxidation potentials smaller than sensitizing dyes. Such compounds are described, e.g., in U.S. Pat. Nos. 2,313,922, 2,075,046, 2,448,858 and 2,680,686, British Patent 1,230,449 and Belgian Patent 771,168.
However, the sensitivity increasing effects of those supersensitizers is still insufficient, so further increase in sensitivity has been required.
In the meantime; tabular silver halide grains (hereinafter referred to as "tabular grains") have photographic characteristics as mentioned below:
1) As tabular grains are great in the ratio of surface area to volume, a large quantity of sensitizing dye can be adsorbed to the grain surface; as a result, higher color sensitization sensitivity can be obtained. PA1 2) When an emulsion comprising tabular grains is coated on a support and dried, the grains are oriented in parallel with the support surface; as a result, the coated layer can have a reduced thickness to enhance the sharpness. PA1 3) As the tabular grains oriented parallel to the support keep their shape and orientation even after development, the developed silver can have high covering power. This characteristic enables further reduction in the amount of coated silver which is required for attaining a definite photographic density, particularly in X-ray films. PA1 4) The tabular grains oriented parallel to the support cause slight scattering of light, so they can provide an image of high resolution, and PA1 5) When used in a green-sensitive or red-sensitive layer, the tabular grains enable the emulsion to reduce or to remove yellow filter because they have low sensitivity to blue light. PA1 (1) A silver halide photographic material comprising a silver halide emulsion layer containing at least one compound selected from the group consisting of compounds represented by the following formulae (I), (II), (III) and (IV) wherein an average aspect ratio of silver halide emulsion grains constituting said silver halide emulsion layer is from 8 to 100: ##STR2## PA1 wherein Z.sub.1 represents atoms completing a 5- or 6-membered nitrogen-containing heterocyclic ring, L.sub.1, L.sub.2, L.sub.3 and L.sub.4 each represent a methine group, V.sub.1 represents a monovalent substituent, l.sub.1 is an integer of from 0 to 4, p.sub.1 is 0 or 1, n.sub.1 is 0, 1, 2 or 3, R.sub.1 represents a hydrogen atom, an alkyl group, an aryl group or a heterocyclic group, M.sub.1 represents a counter ion for adjusting the electric charge balance, m.sub.1 is the number of counter ions necessary to render the molecule electrically neutral and ranges from 0 to 10, and La represents a methylene group; ##STR3## PA1 wherein Z.sub.1, L.sub.1, L.sub.2, L.sub.3, L.sub.4, R.sub.1, p.sub.1 n.sub.1, M.sub.1, m.sub.1 and La have the same meanings as those in formula (I) respectively, R.sub.2 has the same meaning as R.sub.1, V.sub.2 has the same meaning as V.sub.1 in formula (I), and l.sub.2 is an integer of from 0 to 3; ##STR4## PA1 wherein Z.sub.1, L.sub.1, L.sub.2, L.sub.3, L.sub.4, R.sub.1, R.sub.2, V.sub.1, l.sub.1, p.sub.1, n.sub.1, M.sub.1, m.sub.1 and La have the same meanings as those in formula (I) or(II) respectively, provided that at least either L.sub.3 or L.sub.4 has La--CO.sub.2- as a substituent; and ##STR5## PA1 wherein Z.sub.1, L.sub.1, L.sub.2, L.sub.3, L.sub.4, R.sub.1, R.sub.2, V.sub.1, l.sub.1, p.sub.1, n.sub.1, M.sub.1, m.sub.1 and La have the same meanings as those in formula (I) or (II) respectively, provided that the nitrogen-containing heterocyclic ring completed by Z.sub.1 has at least one La--CO.sub.2.sup.- as a substituent. PA1 (2) A silver halide photographic material according to the above embodiment (1), wherein said silver halide emulsion grains are emulsion grains spectrally sensitized with a sensitizing dye. PA1 (3) A silver halide photographic material according to the above embodiment (1) or (2), wherein said silver halide emulsion is a tabular grain emulsion prepared by feeding an aqueous solution of water-soluble silver salt and an aqueous solution of water-soluble halide into a mixing vessel arranged separately from a reaction vessel for carrying out a nucleation process and/or a growth process, stirring and mixing the aqueous solutions in the mixing vessel, thereby forming fine grains of silver halide, and feeding immediately the formed fine grains into the reaction vessel and making them undergo nucleation and/or growth in the reaction vessel. PA1 (4) A silver halide photographic material according to the above embodiment (3), wherein the mixing vessel is provided with (i) a closed stirring tank having at least the desired number of inlets for feeding an aqueous solution of water-soluble silver salt and an aqueous solution of water-soluble halide and an outlet for discharging the silver halide fine grain emulsion formed upon completion of the stirring and (ii) a stirring means that is arranged inside the stirring tank and has no shaft passing through the tank wall but has at least one stirring blade which is driven into rotating to control a stirred condition of the liquid in the tank. PA1 (5) A silver halide photographic material according to any one of the above embodiments (1) to (4), wherein the silver halide emulsion comprises an emulsion made in the presence of a gelatin having the carboxyl groups introduced in a ratio of at least one carboxyl group to one primary amino group by chemical modification of primary amino groups present therein. PA1 (6) A silver halide photographic material according to any one of the above embodiments (1) to (5), wherein the silver halide emulsion is an emulsion prepared going through (a) a process of forming silver halide nuclei comprising twinned microcrystals in a dispersing medium under a condition that the chlorine content in the silver halide nuclei is at least 10 mole % to the silver content therein, (b) a process of ripening the silver halide nuclei so that tabular nuclei remain preferentially, and (c) a process of forcing the tabular nuclei to grow into tabular grains. PA1 (7) A silver halide photographic material according to any one of the above embodiments (1) to (6), wherein the methylene group represented by La in formulae (I) to (IV) is a methylene group substituted with a substituted or unsubstituted alkyl group. PA1 (8) A silver halide photographic material according to any one of the above embodiments (1) to (6), wherein the compound contained in the silver halide emulsion layer is a compound represented by formula (I) in which the La is a methylene group substituted with a substituted or unsubstituted alkyl group.
In U.S. Pat. No. 4,439,520 is described the color photographic material that undergoes improvements in sharpness, sensitivity and graininess by using tabular grains having a thickness less than 0.3 .mu.m, a diameter of at least 0.6 .mu.m and an aspect ratio of at least 8 in at least either a green-sensitive layer or a red-sensitive layer. The term aspect ratio as used herein refers to the ratio of the thickness to the diameter of a tabular grain. Additionally, the expression "diameter of a tabular grain" means the diameter of a circle having the same area as the projected area of a tabular grain determined by observing tabular grains in an emulsion under a microscope or electron microscope. And the expression "thickness of a tabular grain" means the distance between two parallel surfaces forming a tabular grain.
The photographic element comprising silver bromide or iodobromide tabular grains having an average diameter of 0.4-0.55 .mu.m and an aspect ratio of at least 8 is described in U.S. Pat. No. 4,693,964. And the tabular grains having an average diameter of 0.5 .mu.m and a thickness of 0.04 .mu.m are described in Examples of the patent cited above. In addition, the photographic element comprising silver bromide or iodobromide tabular grains having an average diameter of 0.22-0.55 .mu.m and an aspect ratio of at least 8 is described in U.S. Pat. No. 4,672,027. And in Examples of this U.S. Patent are described the tabular grains having a thickness of 0.04 .mu.m.
Further, U.S. Pat. No. 5,250,403 discloses the color photographic element that contains in a minus blue (green and/or red) layer tabular grains having (111) major surfaces, an average diameter of at least 0.7 .mu.m and an average thickness of less than 0.07 .mu.m. Hereinafter, the tabular grains having an average thickness of less than 0.07 .mu.m are referred to as "very thin" tabular grains. In the above-cited U.S. Patent, there are descriptions such that a very thin tabular grain emulsion is attractive by its sensitivity-graininess relationship and, ensuring as it does high image sharpness, the use of such an emulsion in a color photographic element, particularly in a minus blue recording emulsion layer, is favorable.
European Patent 362,699 discloses tabular grains which have a ratio of at least 0.7 between their aspect ratio and their diameter. And the Example of this European patent describes the preparation of tabular grains having a thickness of 0.04 .mu.m.
Thus, studies have so far converged on development of tabular grains further increased in aspect ratio and further decreased in thickness for the purpose of bringing good features of tabular grains into full play. On the other hand, desires for photographs of higher quality are still strong, and it has been required to develop the art of further heightening the sensitivity.
As mentioned above, the surface area of a tabular grain bears a great ratio to the volume thereof, and a large quantity of sensitizing dye can be adsorbed to the grain surface to enable the achievement of higher color sensitization sensitivity. Therein, it is thought that increasing the absorptivity of a sensitizing dye can increase the efficiency in transferring light energy from the sensitizing dye to silver halide to achieve the enhancement of spectral sensitivity.
However, the amount of sensitizing dye adsorbed to the silver halide grain surface has its limit, and is difficult to increase beyond the amount required for single-layer saturated adsorption. Therefore, it is expected to find the art of further increasing the sensitivity.